Particle beam devices have already been in use for a very long time, in order to obtain knowledge about the characteristics and behavior of samples in specific conditions. One of these particle beam devices is an electron beam device, in particular a scanning electron microscope (also referred to in the following text as an SEM).
In the case of an SEM, an electron beam (also referred to in the following text as the primary electron beam) is generated by a beam generator, and is focused by a beam guidance system, in particular an objective lens, onto a sample to be examined. The primary electron beam is passed over a surface of the sample to be examined, in the form of a raster, by a deflection device. The electrons in the primary electron beam in this case interact with the material of the sample to be examined. The interaction results in particular in interaction particles. In particular, electrons are emitted from the surface of the sample to be examined (so-called secondary electrons), and electrons are scattered back from the primary electron beam (so-called back-scattered electrons). The secondary electrons and back-scattered electrons are detected, and are used for image production. This therefore results in an image of the surface of the sample to be examined.
It is also known from the prior art for combination devices to be used to examine samples, in which both electrons and ions can be passed to a sample to be examined. By way of example, it is known for an SEM to additionally be equipped with an ion beam column. An ion beam generator which is arranged in the ion beam column is used to produce ions, which are used for preparation of a sample, (for example removal of a surface of the sample or application of material to the sample), or else for imaging. In this case, the SEM is used in particular to observe the preparation, or else for further examination of the prepared or unprepared sample.
In addition to the already mentioned image production, it is also possible to analyze the energy and/or the mass of interaction particles in more detail. For example, a method is known from mass spectrometry in which secondary ions are examined in more detail. The method is known by the abbreviation SIMS (secondary ion mass spectrometry). In this method, the surface of a sample to be examined is irradiated with a focused primary ion beam. The interaction particles produced in the process, and which are in the form of secondary ions emitted from the surface of the sample, are detected in an analysis unit, and are examined by mass spectrometry. In the process, the secondary ions are selected and identified on the basis of their ion mass and their ion charge, thus allowing conclusions to be drawn about the composition of the sample.
The sample to be examined is irradiated with the focused primary ion beam in known particle beam devices in vacuum conditions (10−3 mbar (10−1 Pa) to 10−7 mbar (10−5 Pa)), generally using a hard vacuum of 10−6 mbar (10−4 Pa). The secondary ions are also examined in a hard vacuum in the analysis unit. Since the secondary ions have a broad kinetic-energy distribution, it is, however, disadvantageous for the secondary ions to be injected directly into the analysis unit. An intermediate unit is required, which transmits the secondary ions to the analysis unit and which reduces the width of the kinetic-energy distribution before the secondary ions are injected into the analysis unit.
An apparatus for transmission of energy of a secondary ion to gas particles is known from the prior art. This apparatus has a container with an internal area in which a damping gas is located. The container is provided with a longitudinal axis, along which a first electrode, a second electrode, a third electrode and a fourth electrode extend. The first electrode, the second electrode, the third electrode and the fourth electrode are each formed from a metal bar. They form a quadrupole unit, which produces a quadrupole alternating field in the container.
The secondary ions generated by an ion beam are introduced into the container and transmit a portion of their kinetic energy to the gas particles by impacts. In order to achieve a sufficiently high impact rate for energy reduction, there is a soft vacuum in the region of 5×10−3 mbar (5×10−1 Pa) in the container. The mean free path length of the secondary ions in the soft vacuum is in the millimeter range. The higher the partial pressure of the gas is in the container, the greater is the impact rate, and accordingly also the capability to transmit energy from the secondary ions to the gas particles. After passing through the container, the secondary ions should have only thermal energy.
The kinetic energy of the secondary ions can be subdivided on the one hand into a radial component and on the other hand into an axial component. The radial component causes the secondary ions to diverge from one another radially with respect to the longitudinal axis of the container. This divergence is reduced in the prior art by the abovementioned quadrupole unit. The quadrupole unit causes the secondary ions to be stored radially in an alternating field along the longitudinal axis of the container. The quadrupole alternating field is therefore a storage field. In principle, the quadrupole unit acts like a Paul trap, in which restoring forces act on the secondary ions.
It is likewise known for the secondary ions not to be stored statically within the container which is provided with the quadrupole unit, but to oscillate harmonically, and this is referred to in the following text as macro-oscillation. In order to store the secondary ions securely in the quadrupole unit, a suitable storage force (FStore) should be provided by the quadrupole alternating field, which is proportional to the ratio of the amplitude of the quadrupole alternating field (UQuad) to a frequency of the quadrupole alternating field (fQuad). Therefore:
                              F          Store                ∼                              U            Quad                                f            Quad                                              [        1        ]            
It is also known for the macro-oscillation to have a further oscillation in the form of a micro-oscillation superimposed on it, at the frequency of the quadrupole alternating field. The micro-oscillation has an amplitude (ZMicro) which is proportional to the ratio, of the amplitude of the quadrupole alternating field (UQuad) to the square of the frequency of the quadrupole alternating field (fQuad).
                              Z          Micro                ∼                              U            Quad                                              (                              f                Quad                            )                        2                                              [        2        ]            
In order to avoid secondary ions being lost by the secondary ions striking one of the abovementioned electrodes of the quadrupole unit, an overall oscillation amplitude, which is the sum of the amplitude of the macro-oscillation and the amplitude of the micro-oscillation, should remain less than the radius of the internal area of the container into which the secondary ions have been introduced.
The amplitude of the macro-oscillation can be reduced by transmitting a sufficiently large amount of energy from the secondary ions to the gas particles. In contrast, the amplitude of the micro-oscillation can be reduced by increasing the frequency of the quadrupole alternating field. However, this reduces the restoring forces acting on the secondary ions in the container, as a result of which a greater quadrupole alternating field amplitude is required in order to store the secondary ions securely in the container.
The impacts of the secondary ions with the gas particles reduce the radial component of the kinetic energy, as a result of which the amplitude of the macro-oscillation is reduced, and the secondary ions are focused on the longitudinal axis of the container.
The axial component of the kinetic energy ensures that the secondary ions pass through the container along the longitudinal axis of the container in the direction of the analysis unit. The abovementioned impacts also reduce the axial component of the kinetic energy, however, as a result of which the energy of some secondary ions will no longer be sufficient to pass through the container completely as far as the analysis unit. In the prior art, a potential gradient is therefore provided on the container, wherein a potential associated with that point is provided at each point on the longitudinal axis. The secondary ions are moved axially in the direction of the analysis unit by the potential gradient. The potential gradient is configured such that the potential decreases continuously in the direction of the analysis unit, and has a potential well in the area of one end of the container, which is directed at the analysis unit. The secondary ions pass through the container and in the process transmit their energy to the gas particles, until they rest in the potential well.
The known quadrupole unit is subdivided into segments in order to produce the potential gradient. Expressed in other words, the first electrode, the second electrode, the third electrode and the fourth electrode are each subdivided into segments. Each segment has a segment length which is sufficiently short that the field punch-through of the potential is also still sufficiently effective in the center of the individual segments. It has been found that the abovementioned occurs when the segment length corresponds substantially to the core radius of the container. The expression core radius may refer to the radius of the internal area of the container within which the secondary ions can move without striking the abovementioned electrodes.
The abovementioned container has a first end and a second end. An inlet is arranged at the first end, through-which the secondary ions enter the internal area of the container from the area in which the secondary ions are generated, and which area is kept in hard-vacuum conditions. A pressure stage is arranged at the inlet. This means an apparatus which separates a first pressure area (in this case a hard vacuum, for example in a sample chamber) from a second pressure area (in this case a soft vacuum in the internal area of the container), such that the vacuum in the first pressure area does not substantially deteriorate. An outlet is provided at the second end of the container, through which the secondary ions leave the container in the direction of the analysis unit. A further pressure stage is arranged at the outlet, which separates the second pressure area (in this case the soft vacuum in the internal area of the container) from a third pressure area (in this case the hard vacuum in the analysis unit), such that the vacuum in the third pressure area does not deteriorate substantially.
With regard to the abovementioned prior art, reference is made, for example, to DE 10 2006 059 162 A1, U.S. Pat. No. 7,473,892 B2, EP 1 185 857 B1, U.S. Pat. No. 5,008,537, U.S. Pat. No. 5,376,791 and WO 01/04611, which are all incorporated herein by reference. Furthermore, reference is made to US 2009/0294641 and U.S. Pat. No. 5,576,540, which are also incorporated herein by reference.
Analyses have shown that, the configuration of the further pressure stage arranged at the outlet is not trivial. A number of preconditions must be observed. In order to have a good effect as a pressure stage, the terminating plate should have a through-opening which is as small as possible and as long as possible (generally formed by a small core hole), which connects the container to the analysis unit and through which the secondary ions can pass in the direction of the analysis unit. By way of example, if the terminating plate is formed from a conductive material, then the terminating plate acts as an electrostatic lens. It is probable that the secondary ions will be reflected on the terminating plate, attracted to it or neutralized by the terminating plate such that the secondary ions do not pass through the small through-opening to the analysis unit. The radial extent of the through-opening could admittedly be enlarged in order in this way to transfer more secondary ions from the container to the analysis unit. However, this would result in the characteristics of the terminating plate as a pressure stage becoming worse, because the larger the radial extent of the small through-opening is, the greater the extent to which the hard vacuum in the analysis unit deteriorates as a result of the ingress of gas particles from the container into the analysis unit.
It is also unsuitable for the terminating plate to be formed from a non-conductive material, because the terminating plate could become charged when secondary ions strike it and would accordingly produce disturbance fields which would disturb the quadruple alternating field in the container, or would deflect secondary ions. In this case, the effects achieved by the quadruple alternating field would be partially cancelled out again. This is undoubtedly undesirable.
Consideration has also been given to providing the internal area of the container with an axially conically converging structure, with the smallest diameter of this conically converging structure being arranged in the area of the second end of the container. This would reduce the core radius in the container to a very small extent. However, this solution is also disadvantageous, because the conically converging structure is such that the axial component of the kinetic energy of the secondary ions could once again be converted into a radial component of the kinetic energy of the secondary ions, as a result of which the secondary ions would once again carry out macro-oscillations with a greater amplitude. The amplitude of the macro-oscillation and the amplitude of the micro-oscillation can be designed such that the secondary ions are not able to pass through a through-opening in a terminating plate in the form of a pressure stage. Furthermore, analyses have shown that the mechanical embodiment and electrical embodiment of the conically converging structure can be produced only with a large amount of effort.
It is also disadvantageous for the pressure stage to be in the form of a conductive, tubular, relatively long container with a relatively large core diameter. A container such as this has an area in which there is no field, as a result of which the radial component of the kinetic energy can lead to defocusing of the secondary ions.
Accordingly, it would be desirable to specify an apparatus for storage and for focusing of ions, and an apparatus for separation of two pressure areas, which are of simple design, on the one hand allow the ions to be focused as well as possible onto a small radius, and on the other hand have good pressure stage characteristics.